Expandable anode for diaphragm cells

ABSTRACT

The invention describes an anodic structure for electrolysis cell provided with a separator, in particular for a diaphragm chlor-alkali electrolysis cell, consisting of at least one movable surface suited to be put in contact with the separator. The movable surface comprises a component of higher thickness in contact with a porous layer of lower thickness, for instance a thin sheet provided with openings, and is characterised by the fact that a catalytic coating is applied only to the porous layer of lower thickness. The component of higher thickness has a substantially planar development and includes elements capable of promoting the local recirculation of electrolyte.

DESCRIPTION OF THE INVENTION

The production of chlorine and caustic soda from sodium chloridesolutions, the production of aluminium from molten salts and theelectrometallurgy are nowadays the main electrochemical processes ofindustrial relevance. In particular, chlorine-caustic soda (orchlor-alkali in general) electrolysis is carried out on the basis ofthree types of technologies, namely the mercury cathode, diaphragm andmembrane one. The latter type of electrolysis is the most advanced andrepresents since a few years the only option left for the constructionof new plants in view of the lower electrical energy costs and of thenear-zero environmental impact, while the mercury cathode and diaphragmtechnologies survive in already paid-off plants wherein the highervariable costs are balanced at least in part by the lower fixed costs.In order to make the operation of these plants acceptable in a situationof increasing electrical energy prices and of growing concern for humanhealth and environment, a continuous technological improvement has beenobserved in the last years, which in the case of diaphragm technologyhas led to the development of inert fibre-based diaphragms as areplacement of the previously used asbestos and to modifications ofcathode and anode design aimed at decreasing the electrical energyconsumption.

Focusing the attention on the anode design, it has been particularlyobserved the replacement of the so-called “box” anodes withexpandable-type anodes, provided with forced expansion devices,optionally of controlled action.

The “box” anodes, born as replacements of the old graphite anodes ofwhich they substantially maintain the external shape (see for exampleU.S. Pat. No. 3,591,483), consist of a titanium sheet provided withopenings, folded so as to form an empty box (whence the name) shaped asa rectangular base prism. During the electrolyser assembly the anodes,which are secured in a multiplicity of parallel rows to a supporting andelectrical current-distributing base, are intercalated betweencorresponding rows of cathodes, also shaped as flat boxes formed byperforated sheets or metal wire meshes coated with a porous diaphragmconsisting of, as previously said, inert fibres stabilised by a polymerbinder. This operation of intercalation is quite delicate and, in orderto avoid that the diaphragms be damaged by strokes or rubbing againstthe anodes, the same anodes have sensibly lower width than the existinggap between the rows of diaphragm-bearing cathodes. It follows that,during operation, the sensible gap existing between anodes anddiaphragms (6-8 mm indicatively) entails a high voltage, to whichcorresponds a high electrical energy consumption.

To overcome this inconvenience, particularly heavy in times of growingelectrical energy prices, expandable anodes were introduced, againshaped as a flat box, but with the two major surfaces secured toexpanding devices (expanders) consisting of titanium sheets endowed withelasticity. In this way the surfaces have a certain mobility, whichallows them, precisely under the push of the expanders, to get closer orfarther away while remaining mutually parallel (see for example U.S.Pat. No. 3,674,676). In particular, when assembling the electrolysers,the anodes are maintained in a restrained position by suitableretainers, thereby assuming a reduced width allowing to prevent damagesto the diaphragms. Once positioned the anodes between rows of cathodes,the retainers are extracted leaving the anode surfaces free to expandunder the effect of the intrinsic elasticity of the structure. Theretainer extraction does not present any particular difficulty since inthe phase of anode assemblage between the rows of cathodes, theelectrolyser is free of cover, and thus the access to the anodes isentirely free.

Notwithstanding that the expansion of this kind of anode might beadjusted so as to bring the major surfaces consisting of sheets providedwith apertures and catalytic coating in direct contact with thediaphragm surfaces, in the industrial practice it is preferred tomaintain a gap of about 3 mm, substantially to prevent the damages whichcould be produced by the contact between the inevitable asperities ofthe sheets, whose openings are cheaply obtained by mechanical expansion,and the diaphragm whose hardness is rather modest. Indicatively, theexpanded sheets are obtained making use of 1 mm thick titanium sheets asthe starting material, and the expansion is adjusted so as to produceopenings of typical rhomboidal shape whose diagonals are about 10 and 15mm long. The thickness of 1 mm for the starting sheet is needed toensure a sufficient electrical conduction, and hence a homogeneouscurrent distribution: in its turn, such thickness of 1 mm imposes, forthe mechanical expansion, the above seen dimensions of the rhomboidalopening diagonals. With these dimensions, a partial penetration of theanode surfaces into the diaphragms cannot be avoided, leading to afurther damaging to their integrity. The safety distance of about 3millimetres is commonly ensured by introducing spacers in the form ofplastic rods or buttons between the movable surfaces of the anodes andthe diaphragms.

The anodes are further provided with brine recirculation means havingthe purpose of favouring the mass transport of chlorides toward theanode surface catalytic coating in order to facilitate the evolution ofchlorine while hindering that of oxygen, which is a typical by-productof reaction. These recirculation means consist of suitable internalducts created by sheets introduced within the “box”-type anodes, asdisclosed in U.S. Pat. No. 4,138,295, or by an adequate shaping of theexpanders, as proposed in U.S. Pat. No. 5,593,555 precisely forexpandable-type anodes. In U.S. Pat. No. 5,066,378 a particularlyefficient recirculation device is presented, comprising bafflesinstalled on top of the expandable anodes and connected to downcomingpipes positioned inside the anodes themselves: the baffles, whichproduce a rapid coalescence of the chlorine bubbles generated upon thecatalytic coatings, allow to achieve a quick degassing withoutgenerating foams and therefore enhance the intensity of brinerecirculation.

The finite distance between the surfaces of the diaphragms and those ofthe anodes after expansion entails a higher cell voltage due to theohmic drop inside the brine film just located between such surfaces:assuming that the gas content in such film doesn't affect the electricalresistivity of brine to a substantial extent, it can be determined thatto a gap of 3 mm (imposed as said above by the spacers of common use inthe industrial practice) corresponds a ohmic drop of about 0.1 Volts.The real ohmic drop results considerably higher and close to 0.3 Voltswhen the diaphragm is of the type based on asbestos fibres mechanicallystabilised with perfluorinated-type polymeric binders, still largely inuse. This seriously negative situation is originated by the remarkableswelling undergone by these types of diaphragm when they are put incontact with an aqueous solution, in particular with the process brineat temperatures around 80-100° C.: the diaphragm practically comes tooccupy a consistent portion of the anode-diaphragm gap and therefore thevolume of brine through which the ionic migration takes place resultssubstantially decreased, with a consequent increase of the ohmic dropfor a given applied current. Although still present, the phenomenon islower in the case of the more modern diaphragms in which, as said above,the asbestos fibres are replaced with inert fibres, for instance made ofperfluorinated polymeric material and inorganic oxide such as zirconiumoxide.

To eliminate or in any case reduce the above indicated ohmic drop,devices suited to compress the diaphragm so as to force it to maintainits initial thickness practically unaltered were perfected. Inparticular, U.S. Pat. No. 5,534,122 disclosed additional elasticelements which are provided to be inserted within the expandable anodes:the purpose of the finding is to impart a force of compression to themovable anode surfaces sufficient to compensate for the push exerted bythe diaphragms in the swelling phase, thereby preventing the thicknessincrease. The problem of damaging the delicate diaphragm under theaction of the compression exerted, as seen above, by the surfacesconsisting of expanded sheet, inevitably quite coarse, is solved byresorting to a composite structure comprising an expanded flattenedsheet of low thickness, for example 0.3-0.5 mm, for which the mechanicalworking for its expansion allows to easily produce rhomboidal openingswith diagonals of reduced length, for instance 3 and 5 mm, fixed to thesheet of higher thickness and wider openings of the prior art. Thecatalytic coating is applied to both of the expanded sheets or at leastto the thin sheet. With the composite structure, a specific role isassigned to the two component sheets. In particular, the thin sheetprovided with small sized openings has the purpose of blocking thediaphragm swelling by applying a much more distributed compressionforce: this feature, together with the regularity of the surface,substantially free of asperities as a consequence of the flatteningtreatment, guarantees that no significant damage is produced to thediaphragms. To the unflattened thicker sheet is deputed the task ofoptimally distributing the electric current while preventing thedeformations which would be unavoidable with the thin sheet alone.Diaphragm chlor-alkali electrolysis cells operating at a current densityof 2000 A/m² and equipped with anodes provided with the compositestructure just described and with the means for brine recirculation, forinstance according to U.S. Pat. No. 5,534,122, are characterised by avoltage 0.1-0.15 Volts lower than the voltages of cells provided withanodes and spacers according to the prior art. Such certainlyinteresting result is nevertheless inferior to the previously indicatedexpected value of about 0.3 Volts.

In U.S. Pat. No. 4,013,525 it is described an anode consisting ofvertical plates having a width of 7 mm, mutually spaced apart by 4 mm:the voltage of a diaphragm chlor-alkali cell equipped with this type ofanode and working at 2000 A/m² results to be about 0.3 Volts better thanthe voltages of cells based on the current technology. A gap of 1.5-3 mmis maintained from the diaphragm, apparently to prevent the mechanicaldamaging produced by the contact with the edges of the plates. Such gapand the rather relevant distance between the plates do not hinder theswelling of the diaphragm in operation: therefore, at least inprinciple, it is legitimate to assume that the voltages of U.S. Pat. No.4,013,525 might be further improved.

An example of optimisation of the structure of a vertical anode withsurfaces consisting of a multiplicity of parallel plates is given byU.S. Pat. No. 4,642,173 and EP 0 203 224.

U.S. Pat. No. 4,642,173, disclosing an anode for specific use inelectrometallurgy cells normally free of separator, takes particularlyinto consideration the positive effect generated by the electrolysissurface amplification given by the surface of the catalyst-coated platesfor a given external bulk: in particular, it is claimed that suchamplification per unit external bulk area be comprised between 4 and 20,and preferably between 6 and 14.

In EP 0 203 224 it is described an equivalent electrode structureconsisting of vertical parallel plates whereof an optimisation isproposed relating both to the amplification of the catalyst-coatedsurface and to the withdrawal of the gas evolved in operation: toachieve the latter objective, it is indicated that the ratio between thewidth of the plates and the distance thereof must be respectivelymaintained between 0.8 and 0.6 and between 0.2 and 0.7. In particular,the use of plates of 1-2 mm thickness and 3-5 mm width is provided. Theelectrode may be employed in many electrochemical applications and inparticular as anode (and optionally as cathode) in diaphragm cells, withthe plate edges positioned, as already seen, at a certain distance fromthe diaphragm to prevent mechanical damages.

A similar structure suitable for working as anode in membranechlor-alkali cells (which can however be employed also in diaphragmcells) is presented in U.S. Pat. No. 5,290,410: in this case thestructure comprises a multiplicity of vertical bars provided withcatalytic coating and welded on a conductive support provided withopenings for the free passage of brine: in the preferred embodiment thebars are rods of diameter comprised between 0.1 and 3 mm, spaced apartby distances of 0.5-2 times the diameter. In the text it is specificallyindicated that the anode may be placed in direct contact with theion-exchange membrane.

In U.S. Pat. No. 4,469,577 it is proposed a structure consisting ofplates of suitable profile disposed horizontally and slanted from thevertical plane defined by the structure itself, provided with catalyticcoating. This particular kind of electrode was conceived to allow anadequate expansion of the ion-exchange membranes with which it mightcome in contact in membrane chlor-alkali cells. It is apparent that thiselectrode, likewise the types just described, cannot be placed in directcontact with diaphragms which it would penetrate under the push of theexpansion devices producing mechanical damages with its edges.

From the above reported analysis of the prior art it is clear that thetypes of known electrode structures, developed with the purpose ofamplifying the surfaces provided with catalytic coating and of promotingthe withdrawal of the product gases and the recirculation of theelectrolytic solutions, are not suitable for being installed as anodesin diaphragm chlor-alkali cells when the scope is minimising theproduction energy consumption (kWh/ton of chlorine).

This purpose is at the basis of the instant invention, which under afirst aspect is directed to an electrode for cells provided withseparator and particularly, although not exclusively, for diaphragmchlor-alkali cells and more particularly to an anodic structure providedwith at least one movable surface suited to be installed in diaphragmchlor-alkali cells in direct contact with the diaphragm itself withoutany risk of mechanical damages.

In a second aspect of the invention, the movable surface is providedwith elements capable of inducing an effective local recirculation ofbrine.

In a third aspect the invention discloses an anode for cells providedwith separator, particularly for diaphragm chlor-alkali cells, capableof ensuring a substantial reduction of the electrolysis voltage.

In a fourth aspect the anode of the invention is characterised byproducing chlorine with lower oxygen contents.

In a final aspect the anode of the instant invention is characterised byreduced electric energy consumption per tonne of product chlorine.

Thus, according to present claim 1, the invention concerns an electrodestructure for electrolysis cell divided by a separator into an anodiccompartment and a cathodic compartment, comprising at least one movablesurface suited to be put in contact with the separator and provided witha thicker component and a thinner component overlapped thereto, thethicker component being generally planar and the thinner component beinga thin sheet provided with openings or a thin mesh of wires. Theelectrode structure of the invention is characterised in that only thethicker component is provided with a catalytic coating. Contrary toprior art, only the underlying thicker component is catalyst-coatedwhile thinner component, i.e. the outermost component directly facingthe separator, remains uncoated.

Other preferred embodiments of the electrode structure of the inventionare defined in the dependent claims.

The invention also concerns a chlor-alkali membrane or diaphragm cellcomprising at least one electrode structure of the invention.

Finally, the invention concerns a chlor-alkali electrolysis processcarried out in the such a cell, the process being characterised byhaving a voltage not higher than 3 Volts at a current density of 2500A/m² and an oxygen content in chlorine non higher than 2%.

These and other peculiar aspects of the invention are discussed in thefollowing presentation.

DESCRIPTION OF THE FIGURES

The present invention is described making reference to the figureslisted below:

FIG. 1 a: three-dimensional view of anode in accordance with theinvention with each of the two major movable surfaces consisting of acomponent of higher thickness and substantially planar developmentresulting from a multiplicity of horizontal plates slanted with respectthe vertical plane and by a further thin porous layer, for instance inform of perforated sheet, expanded sheet, mesh of wires, layer ofsintered material, applied on the outer edge of the multiplicity ofplates.

FIG. 1 b: side-view of section along line X-X of FIG. 1 a.

FIG. 1 c: top-view of section along line Y-Y of FIG. 1 a.

FIG. 1 d: three-dimensional view of the current collecting stem alone,provided with expanders.

FIG. 2 a: front-view of a particular embodiment of the plate structureof figure 1 a.

FIG. 2 b: side-view of section along line W-W of FIG. 2 a

FIG. 3 a: three-dimensional view of anode in accordance with theinvention with each of the two major movable surfaces consisting of acomponent of higher thickness and substantially planar developmentresulting from a multiplicity of vertical plates and by a further thinporous layer, for instance in form of perforated sheet, expanded sheet,mesh of wires, layer of sintered material, applied on the outer edge ofthe multiplicity of plates.

FIG. 3 b: side-view of section along line K-K of FIG. 3 a.

FIG. 3 c: top-view of section along line Z-Z of FIG. 3 a.

FIG. 4 a: three-dimensional view of a further embodiment of the anode ofthe invention with each of the two major movable surfaces consisting ofa thicker porous sheet, for instance a perforated sheet, expanded sheet,mesh of wires, layer of sintered material, with a second thin poroussheet applied thereto, also in form of perforated sheet, expanded sheet,mesh of wires, layer of sintered material.

FIG. 4 b: side-view of section along line S-S of FIG. 4 a.

FIG. 4 c: top-view of section along line T-T of FIG. 4 a.

DETAILED DESCRIPTION OF THE INVENTION

Although the finding is represented as an electrode structure suitablefor being advantageously installed as anode and/or cathode on severalkinds of cells provided with separator, for the sake of highersimplicity of description but without wishing to limit thereby thescopes of the present invention, reference will be made in the followingto the use of this structure as anode in diaphragm chlor-alkali cells,which constitute a field of application of remarkable industrialinterest.

The inventors, in their quest for an anode structure suited tofunctioning in diaphragm chlor-alkali cells at low voltage, reducedoxygen content in chlorine and lower energy consumption per tonne ofproduct, and without diaphragm damaging hazard, have tested the noveltypes of anodic structure described in the following. The first type ofstructure is represented in a three-dimensional view in FIG. 1 a,wherein (1) indicates the current collecting stem consisting of a coreof highly conductive metal such as copper provided with an externallayer of corrosion resistant metal such as titanium, niobium, tantalum,(2) the foot of the stem provided with a threaded portion to allow thefixing on the supporting anodic sheet (not represented), (3) theexpanders consisting of elastic elements which allow to maintain the twomajor surfaces in a restrained position, that is adherent to the currentcollecting stem, during the cell assemblage and to bring them to anexpanded position, that is apart from the current collecting stem and indirect contact with the diaphragm surface (not shown) during operationas known to the experts of the field, (4) the multiplicity of parallelhorizontal plates which are secured to supporting bars (6) secured intheir turn to the edges of the expanders forming one of the two majorsurfaces, the other surface being schematised by the contour (10), (5)the thin porous layer consisting of an expanded flattened sheet fixed,for instance by welding, to the edges of the plates (4). In the detailsof FIGS. 1 b and 1 c are sketched the two sections of the anode of FIG.1 a along the lines X-X and Y-Y respectively as side-view and astop-view. For a better understanding, in FIG. 1 d the current collectingstem (1) provided with terminal part (2) and expanders (3) without theirmajor surfaces is represented in three dimensions. For a betterfunctioning, the anode of FIG. 1 a is preferably provided with theadditional expanding elements disclosed in U.S. Pat. No. 5,534,122. InFIGS. 2 a and 2 b it is shown a particular embodiment of themultiplicity of plates of FIG. 1 a, respectively as a front-view and asa side-view of the section along line W-W. In this embodiment, thehorizontal plates are obtained by making cuts of suitable length inparallel and off-set horizontal rows on a sheet (7), and by subsequentlydeforming the sheet in correspondence of the cuts in order to form themultiplicity of plates known as “louver geometry”. The advantage of thisstructure is given by the very quick fabrication procedure which doesnot require the assembling of separate plates. On the sheet providedwith openings it is secured the expanded flattened thin sheet (5), asalready seen for the anode of FIG. 1 a. The assembly of sheet (7) andthin sheet (5) is in its turn fixed to the expanders (not represented)as seen in the case of the anode of FIG. 1 a.

FIG. 3 d reproduces a three-dimensional view of an embodiment of anodicstructure, wherein (1) indicates as already seen the current collectingstem provided with threaded foot (2) for the fixing to the anodicsupporting sheet (not represented), (3) the expanders, (8) amultiplicity of vertical plates supported by the horizontal bars (9),secured to the supporting bars (6), in their turn fixed to the expanders(3); once again, the other major surface is schematised by the contour(10). The multiplicity of vertical plates (8) finally supports theexpanded flattened thin sheet (5). FIGS. 3 b and 3 c respectively show aside-view and a top-view of the two sections of FIG. 3 a along the linesK-K and Z-Z.

Finally, FIG. 4 a reports a three-dimensional view of a furtherembodiment of anodic structure wherein the common parts to the previousembodiments are indicated with the same identifying numerals: thecomponent of higher thickness consists of a sheet provided with openings(11) secured, for instance by welding, to the above seen thin sheet (5).FIGS. 4 b and 4 c respectively show a side-view and a top-view of thetwo sections of FIG. 4 a along the lines S-S and T-T.

The above disclosed anode structures were installed in lab diaphragmcells having an active area of 13 centimetre width and 100 centimetrelength, equipped with the diaphragms based on asbestos fibre stabilisedby polytetrafluoroethylene as binder deposited on a cathode consistingof a mesh of carbon steel wires disclosed in the examples of U.S. Pat.No. 5,534,122. The cells were operated at a current density of 2500A/m², at 90-95° C., with a purified brine feed containing 315 g/l ofsodium chloride and 0.5 mg/l of calcium+magnesium and with an outletelectrolyte containing about 125 g/l of caustic soda and about 190 g/lof residual sodium chloride. The anodic structures employed had thefollowing geometrical features:

Type A: horizontal plates of the type shown in FIG. 2 a, obtained bymaking cuts 15 millimetre long on a 1 millimetre thick sheet, inparallel and off-set horizontal rows spaced apart by 2.5 millimetres andthen deforming the thus pre-cut sheet in correspondence of each of thecuts, so as to form a multiplicity of plates according to the geometryknown as “louvered”, with the plates slanted by 30° with respect to thevertical plan. Thin sheet obtained from 0.5 millimetre thick sheet,expanded and flattened with formation of rhomboidal openings havingdiagonals of 3 and 5 millimetres.

Type B: vertical plates of the type shown in FIG. 3 a, 4 millimetrewide, 1 millimetre thick, spaced apart by 4 millimetres. Thin sheetequivalent to that used in the anodic structure of type A.

Type C used as reference structure, consisting, in accordance with thedisclosure of U.S. Pat. No. 5,534,122, of the overlap of a thin sheetequivalent to the one employed for types A and B on a sheet obtained byexpansion without flattening of a 1 millimetre thick sheet withrhomboidal openings having diagonals respectively of 10 and 15millimetres (FIG. 4 a).

The catalytic coating employed, based on the formulation commonly usedfor diaphragm chlor-alkali cell anodes and consisting of a mixture ofruthenium and titanium oxides, was applied only to the thin sheet(anodes A1, B1), to both the thin sheet and the plates (anodes A2, B2),to the plates alone (anodes A3 e B3). As concerns type C, the coatingwas applied at least to the thin sheet (C1) or to both sheets, the thinand the thicker one (C2).

The results obtained can be summarised as follows:

anode A1. Cell voltage: 2.9 Volts slowly rising in time up to 3.2 Voltsafter 250 hours of operation, with no further variation until the end ofthe test (780 hours), oxygen content in chlorine: almost constant around3.5%.

anode A2. Cell voltage: 2.9 Volts slowly rising up to 3.0 Volts after200 hours of operation, with no further variation until the end of thetest (850 hours), oxygen content in chlorine: fluctuating with anaverage value of 3.2%

anode A3. Cell voltage: 3.0 Volts practically constant in the course ofthe test (990 hours), oxygen content in chlorine: about 2%

anode B1. Cell voltage: 2.8 Volts constantly rising up to 3.1 Volts inthe first 200 ore of operation, with no further variation untildisassembling the cell (770 hours), oxygen content in chlorine: about3.3%, with fluctuations of small entity.

anode B2. Cell voltage: 2.8 Volts rising up to 3.0 volt, a value almostunvaried until the end of the test (770 hours), oxygen content inchlorine: about 3.1%

anode B3. Cell voltage: 2.9 Volts practically unvaried in the course ofthe test (1050 hours), oxygen content in chlorine: 1.8%

anodes C1 e C2. Cell voltage: 2.9 Volts with increase up to 3.3 Voltsafter 150 hours with no further sensible variation until the end of thetest (750 hours), oxygen content in chlorine: 3.7%, fairly constant intime.

The situation resulting from the test is in principle quite surprising,since as regards the voltages, the better results should be obtainedwith the anodes of type A2 and B2, clearly provided with a higheroverall surface with catalytic coating. In fact, this occurs with theinitial voltages, which nevertheless increase in time up to valuesessentially similar to those of the cells equipped with anodes of typeA3 and B3. A similar behaviour may perhaps be explained assuming that,under the effect of the pressure exerted by the expanding devices, analbeit moderate penetration of the thin sheet into the diaphragm surfacetakes place: in this way, part of its surface with the relevantcatalytic coating would be blinded. Moreover, the contact betweencoating and diaphragm could introduce some oxygen bubbles inside thediaphragm (chlorine should be absorbed by the alkalinity diffused fromthe cathode side), capable of hindering at least in part the passage ofelectrical current. A further negative factor is probably represented bydisuniformity in the distribution of current, which tends to concentrateinside the diaphragm in correspondence of the meshes of the thin sheetwhen they are in contact with the diaphragm or even partiallypenetrating inside it. In practice after a certain period, depending onthe working conditions and the nature of the diaphragm, the behaviour ofanodes of types A2 and B2 would finally coincide with the one of anodesA3 and B3 wherein the catalytic coating is applied on the plates alone.What said above also allows to understand the functioning of anodes A1and B1 which is clearly the worst: in this case, the fact of havingrestricted the catalytic coating application to the thin sheet aloneentails that the negative effects of blinding and of partial obstructionof the current transmission are maximised, being the compensating actionof the catalytic coating, whereof the plates are free, unable to play arole.

As regards anodes A3 and B3, it would be obvious to expect a relativelyhigh voltage value due to the higher ohmic drop in the brine, since thecatalyst-coated plate surface is placed at a distance from the diaphragmcorresponding to the thickness of the thin sheet. As a matter of fact,the current is redistributed along the lateral surface of the plates sothat its density decreases steeply when the distance from the diaphragmincreases with a parallel decrease of the ohmic drop, which therebyresults substantially lower than the expected values.

The better behaviour of the anodes of family B compared to theequivalent ones of family A is likely an indication of better brinerecirculation with the vertical plate structure with respect to the onewith horizontal plates: the better recirculation determines in fact aquicker brine replacement also in the more blinded areas, with afavourable impact on the overall operating voltage.

Besides the values of voltage, the oxygen contents in chlorine are ofgreat practical interest: oxygen is in fact a by-product whose formationis a useless waste of energy determining as a consequence an energyconsumption increase per tonne of chlorine. Moreover, excessive levelsof oxygen may cause problems in the downstream processes in which thechlorine is employed.

The higher oxygen content in chlorine characteristic of all the anodesA1, A2, B1, B2 may perhaps be explained by remembering that a certainportion of caustic soda migrates back toward the anodic compartmentestablishing a generally alkaline pH profile within the diaphragm,probably capable of extending also to the brine film adhering to thesurface of the same diaphragm. The thin sheet provided with catalyticcoating and kept in contact with the diaphragms by the expanding devicesis practically in direct contact with alkaline brine: it follows afacilitated oxygen evolution up to the relatively high levels recordedduring the test. The oxygen evolution is further enhanced if the thinsheet penetrates albeit partially inside the diaphragm surface.

In the case of anodes A3 and B3 the thin sheet, or equivalent structuresuch as for example a mesh of wires of the same thickness, is free ofcatalytic coating which is only applied to the plates where thealkalinity cannot arrive being dispersed by the local turbulence. Thefact that the oxygen content in chlorine results lower with the B3 typeanode than with the A3 one can be maybe justified by the higher localturbulence supported by the vertical plates with respect to thehorizontal ones.

As said before, the oxygen content in chlorine assumes a particularimportance since it directly influences the oxygen consumption per tonneof product chlorine. In particular, disregarding other factors of yieldloss, such a consumption results of 2300 kWh for anode B3, better typeamong those utilised, to be compared with the consumption of 2450 kWhrelative to anode B2, which represents the second better performance.

As regards anodes C1 and C2 (prior art) the relevant behaviour (energyconsumption: 2650 kWh) is evidently of lesser value than those displayedby types A1, A2, A3, B1, B2 and B3 respectively. This result can be wellinterpreted in view of the working hypotheses previously exposed,particularly in view of the less effective local recirculation supportedby the meshes of the thicker sheet compared to the one characteristic ofthe horizontal and vertical plates. A less efficient recirculationresults in a lower brine replacement in the more blinded areas of thestructure with consequent depletion in chlorides which is the cause ofhigher voltages and higher oxygen content in chlorine.

For the sake of test completion the inventors have also verified thefunctioning of an anodic structure, indicated as C3, equivalent to thatof C1 and C2, with the difference in the application of the catalyticcoating limited to the thicker sheet alone. A cell voltage of 3.1 Voltswas recorded with no sensible variation in the course of the test (800hours) with a slowly rising oxygen content in chlorine from an initial2.4% to a final 2.5%. Thus this anode, even if characterised by notparticularly brilliant performances but also higher than the anodes C1and C2 of the prior art, can be considered for all purposes as anembodiment of the present invention, although less preferred.

In conclusion, the anodes in accordance with the present invention,consisting of a thin porous layer, such as for instance a sheet providedwith openings and flattened, coupled to a porous component of higherthickness capable of promoting the local recirculation of brine, such asfor instance a multiplicity of horizontal or vertical plates, with thecatalytic coating only applied to such component, achieve in asatisfactory manner the objectives initially put forward of low cellvoltage, low oxygen content in chlorine, contact with negligible risksof mechanical damaging with the diaphragms of chlor-alkali cells.

Several variations of the instant invention are possible, as certainlyresults clear to the experts of the field. Some of these are listed forthe purpose of exemplifying:

The advantages characterising the anodes consisting of a thin expandedflattened sheet (or equivalent planar structure such as for instance amesh of wires) and of a structure directed to favour the localrecirculation of brine, are obtainable with just marginal variationsalso when the diaphragms are asbestos-free and consist of inert fibres,for instance made of fluorinated polymer such as polytetrafluoroethyleneand/or of inorganic material such as zirconium oxide, stabilised bybinders chemically resistant to the operating conditions of diaphragmchlor-alkali cells.

The thin sheet or equivalent structure may be made of metal or polymer,preferably hydrophilised to prevent the adhesion of gas bubbles.

The thin sheet or equivalent structure may have a thickness comprisedbetween 0.1 and 1 millimetres, preferably between 0.3 and 0.5millimetres.

The thin sheet may have a ratio between opening clearance and surfaceoccupied by the construction material of at least 50%, preferably of atleast 70%, even more preferably of at least 90%. High values of theratio permit avoiding that local concentrations of current be createdwith an undue ohmic drop increase. Typical sizes, although not limiting,are: width of construction material portions 0.2-0.8 millimetres, forinstance 0.5 millimetres, rhomboidal openings with major and minordiagonal respectively 1 to 5 and 3 to 7 millimetre long, for instance 3and 5 millimetres.

The thin sheet may be disposed directly on the diaphragms instead ofbeing fixed to the anodic structure. In this case the pressure exertedby the anodic structure under the push of the expanding devicesdetermines the intimate contact between the structure itself and thethin sheet, required for the functioning of the anode of the invention.

The structures directed to promote the local recirculation of brinepreferably consist of horizontal or vertical plates whose thickness,spacing and width must be optimised according to the operatingconditions, in particular the current density employed, and to thediaphragm type. In principle, also considering the convenience ofobtaining the highest possible coated surface and the best electricalcurrent repartition, structures consisting of low thickness and lowspacing plates result favourable. To facilitate the construction it ispresumable that the minimum thicknesses should be around 0.3millimetres, and the minimum spacing around 1 millimetre, while thewidth is limited by the admissible width of the anode, which although ina restrained position must be easily insertable between the cell cathodefingers. In the examples, structures comprising 15 millimetre horizontalplates obtained by deformation of a 1 millimetre thick sheet have beendisclosed, but different widths, for instance comprised between 5 and 30millimetres for plates obtained from sheets of thickness comprisedbetween 0.3 and 2 millimetres can be likewise employed. It has been alsodisclosed an optimum spacing of 2.5 millimetres, but values comprisedbetween 1 and 5 millimetres also allow to practise the inventionadvantageously. As regards the structure with vertical plates, 1millimetre thick plates, with width and spacing of 4 millimetres weredisclosed, but thicknesses from 0.3 to 2 millimetres for plates of widthand spacing comprised between 2 and 10 millimetres could also beemployed.

With the industrial size anodes of about 0.7-1 m² per side it isprobably difficult to exert a uniform pressure on the diaphragms whosesurfaces may present planarity defects. To obviate this problem theanode is advantageously subdivided into separate sections, each securedto the expanders: the elasticity of the expanders allows a little tiltwhich facilitates a more uniform contact, and therefore an improvedrepartition of the compression, even with diaphragm characterised bysensible irregularities of planarity.

The present invention is not only relative to only newly constructedanodes, since the structure preferably consisting of the vertical orhorizontal plate panels can be easily installed also on previously usedanodes: the relevant procedure provides the detachment of the oldexpanded sheet whose catalytic coating is exhausted, the cleaning of theterminal parts of the expanders from residues of previous welds, theconstruction of the panels consisting of plates secured to supportingbars and provided with catalytic coating, the welding of the panels tothe terminal parts of the expanders, with a final step represented forinstance by the welding in case the thin sheet is made out of metal.

An entirely equivalent procedure is followed when the newly constructedanodes have lost their catalytic activity after a prolonged operation.As an alternative, it is conceivable to restore the catalytic activityby detaching the thin sheet alone and fixing in its place an expandedsheet of high thickness provided with catalytic coating and thereupon,in a position facing the diaphragms, a thin sheet or equivalent meshfree of catalytic coating. This composite structure corresponds to thetype of electrode previously identified as C3: in this case, however,performances should turn out to be improved by the presence of the platestructure behind which, even with an exhausted catalytic coating, stillpromotes an effective local recirculation, decisive as seen above tokeep the cell voltage and the oxygen content in chlorine at low levels.

The present description shall not be understood as limiting theinvention, which may be practised according to further differentembodiments without departing from the scopes thereof, and whose extentis solely defined by the appended claims.

In the description and claims of the present application, the word“comprise” and its variation such as “comprising” and “comprises” arenot intended to exclude the presence of other elements or additionalcomponents.

1. An electrode structure for an electrolysis cell divided by aseparator into an anodic compartment and a cathodic compartment,comprising at least one movable surface suited to be put in contact withthe separator and provided with a component of higher thickness having asubstantially planar development overlapped to a thin sheet providedwith openings or to a thin mesh of wires, and a catalytic coatingapplied only to said component of higher thickness.
 2. The structure ofclaim 1 characterized by being an anodic structure wherein the separatoris a diaphragm or membrane for a chlor-alkali cell and said catalyticcoating comprises a catalyst for chlorine evolution.
 3. The structure ofclaim 1 wherein said component of higher thickness consists of amultiplicity of vertical and parallel plates.
 4. The structure of claim3 wherein said vertical plates have a width between 2 and 10millimeters, a thickness between 0.3 and 2 millimetres and a spacingbetween 2 and 10 millimeters.
 5. The structure of claim 1 wherein saidcomponent of higher thickness consists of a multiplicity of horizontaland parallel plates.
 6. The structure of claim 5 wherein said plateshave a thickness of at least 0.3 millimeters and are mutually spacedapart by at least 1 millimeter.
 7. The structure of claim 5 wherein saidhorizontal plates are arranged in parallel and in off-set rows spacedapart by 1 to 5 millimeters, each of said plates being 5 to 30millimeter long, said plates being obtained by deformation of a 0.3 to 2millimeter thick sheet.
 8. The structure of claim 1 wherein saidcomponent of higher thickness consists of a sheet provided withopenings.
 9. The structure of claim 8 wherein said sheet provided withopenings is an unflattened expanded sheet.
 10. The structure of claim 1wherein said thin sheet provided with openings is a flattened expandedsheet or a perforated sheet or a sintered porous layer.
 11. Thestructure of claim 10 wherein said flattened expanded sheet is 0.2 to0.8 millimeter thick and is provided with rhomboidal openings with majordiagonal between 3 and 7 millimeters and minor diagonal between 1 and 5millimeters.
 12. The structure of claim 1 wherein said thin sheetprovided with openings has a ratio between opening clearance and overallgeometric area at least equal to 0.5.
 13. The structure of claim 12wherein said ratio between opening clearance and overall geometric areais at least equal to 0.9.
 14. The structure of claim 1 wherein said thinsheet provided with openings is made of a corrosion-resistant metal orof a polymeric material stable at the operating conditions of said cell.15. The structure of claim 14 wherein said corrosion-resistant metalconsists of titanium or titanium alloy.
 16. The structure of claim 14wherein said polymeric material consists of an optionally hydrophilizedfluorinated polymer.
 17. The structure of claim 1 wherein said thinsheet provided with openings or thin mesh of wires is secured to saidcomponent of higher thickness.
 18. The structure of claim 1 wherein saidthin sheet provided with openings or thin mesh of wires is placed incontact with the separator.
 19. A chlor-alkali membrane or diaphragmcell comprising at least a structure of claim
 1. 20-21. (canceled)
 22. Amethod of producing chlorine comprising electrolyzing an aqueous sodiumchloride solution in a cell of claim 19 at a voltage not higher than 3volts at a current density of 2500 A/m² and an oxygen content inchlorine not higher than 2%.